TNT (2,4,6-trinitrotoluene, also called .alpha.TNT), as conventionally manufactured by nitration processes from toluene and mixed acid as starting materials, contains significant proportions (usually about 4%) of undesired, unsymmetrical isomers. The standard industrial procedure for removing these isomeric impurities is to treat the crude TNT with aqueous sodium sulfite ("sellite"), which reacts with the reactive nitro groups in the meta or 3-position and produces a waste sellite solution containing a mixture of sodium 2,4-dinitrotoluene-3-and-5-sulfonates. The aqueous solution of these sulfonates, commonly referred to as "red water" or "spent sellite," constitutes a pollution and disposal problem. According to current practice, the red water is donated to paper companies, which furnace the material to recover the crude sodium salts (essentially sodium sulfate) for use in their operations. However, such procedure pollutes the atmosphere so that there is doubt as to how long the practice will be permitted to continue.
As far as I am aware, no other sulfite has been proposed for purifying TNT aside from the use of ammonium sulfite recently utilized for such purpose, which is the subject of my copending U.S. application Ser. No. 585,735 for "Process For Purifying TNT," filed June 10, 1975.
Magnesium sulfite has long been employed for the treatment of wood pulp in the manufacture of paper. More recently magnesium oxide has been utilized for the removal of sulfur dioxide from combustion gases (International Journal of Sulfur Chemistry, B, Vol. 7, No. 1, 60 (1972)). In both cases the resulting magnesium salts are converted back to magnesium oxide by heating. The magnesium oxide thus obtained is recycled to the process, which in the case of the pulping process involves reconversion to magnesium sulfite by reaction with sulfur dioxide.